Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid catalyst. Old ethylbenzene production plants, typically built before 1980, used AlCl3 or BF3 as the acidic catalyst. Newer plants have in general been switching to zeolite-based acidic catalysts.
Commercial ethylbenzene manufacturing processes typically require the use of polymer grade ethylene, which has a purity exceeding 99.9 mol %. However, the purification of ethylene streams to polymer grade is a costly process and hence there is considerable interest in developing processes that can operate with lower grade ethylene streams. One such ethylene source is the dilute ethylene obtained as an off gas from the fluid catalytic cracking or steam cracking unit of a petroleum refinery which, after removal of reactive impurities, such as propylene, typically contains about 20-80 wt % ethylene, with the remainder being ethane together with minor amounts of hydrogen, methane and benzene.
Three types of ethylation reactor systems are used for producing ethylbenzene, namely, vapor phase reactor systems, liquid phase reactor systems, and mixed phase reactor systems.
In vapor-phase reactor systems, the ethylation reaction of benzene and ethylene is carried out at a temperature of about 380-420° C. and a pressure of 9-15 kg/cm2-g in multiple fixed beds of zeolite catalyst. Ethylene exothermally reacts with benzene to form ethylbenzene, although undesirable chain and side reactions also occur. About 15% of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products. All these chain reaction products are commonly referred as polyethylated benzenes (PEBs). In addition to the ethylation reactions, the formation of xylene isomers as trace products occurs by side reactions. This xylene formation in vapor phase processes can yield an ethylbenzene product with about 0.05-0.20 wt % of xylenes. The xylenes show up as an impurity in the subsequent styrene product, and are generally considered undesirable.
In order to minimize the formation of PEBs, a stoichiometric excess of benzene, about 400-900% per pass, is applied, depending on process optimization. The effluent from the ethylation reactor contains about 70-85 wt % of unreacted benzene, about 12-20 wt % of ethylbenzene product and about 3-4 wt % of PEBs. To avoid a yield loss, the PEBs are converted back to ethylbenzene by transalkylation with additional benzene, normally in a separate transalkylation reactor.
By way of example, vapor phase ethylation of benzene over the crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. Nos. 3,751,504 (Keown et al.), 3,751,506 (Burress), and 3,755,483 (Burress).
In most cases, vapor phase ethylation systems use polymer grade ethylene feeds. Moreover, although commercial vapor phase processes employing dilute ethylene feeds have been built and are currently in operation, the investment costs associated with these processes is high and the products contain high concentrations of xylene impurities.
In recent years the trend in industry has been to shift away from vapor phase reactors to liquid phase reactors. Liquid phase reactors operate at a temperature of about 220-270° C., which is under the critical temperature of benzene (290° C.). One advantage of the liquid phase reactor is the very low formation of xylenes and oligomers. The rate of the ethylation reaction is lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually economically compensates for the negatives associated with the higher catalyst volume. Thus, due to the kinetics of the lower ethylation temperatures, resulting from the liquid phase catalyst, the rate of the chain reactions forming PEBs is considerably lower; namely, about 5-8% of the ethylbenzene is converted to PEBs in liquid phase reactions versus the 15-20% converted in vapor phase reactions. Hence the stoichiometric excess of benzene in liquid phase systems is typically 150-400%, compared with 400-800% in vapor phase.
Liquid phase ethylation of benzene using zeolite beta as the catalyst is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, see, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); International Patent Publication Nos. WO97/17290 and WO01/21562 (ITQ-2).
Commerical liquid phase ethylbenzene plants normally employ polymer grade ethylene. Moreover, although plants can be designed to accept ethylene streams containing up to 30 mol % ethane by increasing the operating pressure, the costs associated with the design and operation of these plants are significant.
Technology has also been developed for the production of ethylbenzene in a mixed phase using reactive distillation. Such a process is described in U.S. Pat. No. 5,476,978. Mixed phase processes can be used with dilute ethylene streams since the reaction temperature of the ethylation reactor is below the dew point of the dilute ethylene/benzene mixture, but well above the bubble point. The diluents of the ethylene feed, ethane, methane and hydrogen, remain essentially in the vapor phase. The benzene in the reactor is split between vapor phase and liquid phase, and the ethylbenzene and PEB reaction products remain essentially in the liquid phase. However, reactive distillation units are complex and expensive and the catalyst is prone to deactivation as a result of the production of ethylene oligomers.
U.S. Pat. No. 6,252,126 discloses a mixed phase process for producing ethylbenzene by reaction of a dilute ethylene stream containing 3 to 50 mol % ethylene with a benzene stream containing 75 to 100 wt % benzene. The reaction is conducted in an isothermal ethylation section of a reactor vessel which also includes a benzene stripping section, where the unreacted benzene is thermally stripped from the ethylation products. Integrated, countercurrent vapor and liquid traffic is maintained between the ethylation section and the benzene stripping section.